1. Field of the Invention
The present invention is broadly concerned with a reliable dump valve assembly to be used as a part of a downhole pipe string employed in oil wells or the like, and to an improved method for selective draining of pumping fluid from the pipe string prior to elevation thereof to the surface, thereby eliminating or minimizing environmental contamination caused by fluid drainage from the pipe string. More particularly, the invention is concerned with such a dump valve assembly and method, which preferably makes use of a metallic, frangible, concavo-convex rupture disc configured for reliably bursting at a predetermined internal fluid pressure. and an improved method of operator control, selective full drainage of pumping fluid.
2. Description of the Prior Art
Conventional oil or gas wells include an annular, cementitious elongated casing extending from grade down to the recoverable oil supply. An elongated, tubular, multiple-section pipe string is positioned within the casing and in turn houses and/or is attached to an artificial lift system.
It is necessary from time to time to extract the artificial lift system or their components for servicing thereof. This involves elevating the pipe string and/or sucker rod assembly, and detaching these parts section by section. However, owing to the fact that the pipe string is filled with pumping fluid, it sometimes occurs that this fluid is simply drained at grade during the dismantling process; this is objectionable because of the environmental pollution it causes.
It has been known in the past to provide dump valve units associated with downhole pipe strings which can be remotely actuated so as to drain the pipe string prior to and during the extraction process. For example, U.S. Pat. No. 4,286,662 describes a tubing drain fitted into the sidewall of a short pipe section coupler which has a blowout piston retained in place by a shear pin. A similar device has been commercialized under the designation xe2x80x9cSeries 500 Tubing Drainxe2x80x9d by Quinn""s Oilfield Supply Ltd. of Red Deer, Alberta.
The present invention provides an improved dump valve assembly adapted for interconnection between a pair of elongated downhole pipe sections forming a part of a downhole pipe string, with the dump valve assembly permitting selective drainage of fluid from the string. The assembly of the invention is relatively inexpensive and affords reliable drainage of a pipe string through the simple expedient of pressurizing the column of pumping fluid within the string to a predetermined pressure (e.g., 2000 psi).
Broadly speaking the dump valve assembly of the invention is operable for interconnecting a pair of string pipe sections in an end-to-end relationship and includes an elongated, tubular pipe section connection body presenting opposed ends in a sidewall; the ends have structure for interconnecting the pair of pipe sections, whereas the sidewall has opposed inner and outer surfaces and is configured to present an opening therethrough. A fitting is located within the sidewall opening and includes a metallic, frangible, shape-retaining rupture disc oriented for normally preventing fluid flow through the sidewall opening. The rupture disc has at least one line of weakness formed therein for reliable rupture thereof upon pressurization of the fluid within the pipe string in order to permit drainage thereof. Preferably, the rupture disc is of concavo-convex configuration and is oriented with the concave face thereof proximal to the inner face of the connection body sidewall; the lines of weakness may be formed by grinding or other ablation processes, but preferably are formed by scoring.
The fitting is normally in the form of an annular metallic housing presenting inboard and outboard surfaces and is adapted for receipt within the sidewall opening, with the rupture disc being fixedly secured to the housing (preferably by permanent welding). The sidewall opening and outboard surface of the housing are complementally threaded for threaded mounting of the housing within the sidewall. Accordingly, it is a simple matter to replace the fitting after each use of the dump valve assembly. The housing also normally includes structure for retaining the disc upon rupture thereof, although in preferred forms the disc is provided with lines of weakness arranged so as to eliminate or minimize the possibility of disc fragmentation upon rupture thereof.
Preferred rupture discs in accordance with the invention includes a pair of intersecting, transverse score lines on the convex face thereof. The disc should normally have a thickness of from about 0.003-0.090 inches, and more preferably from about 0.010-0.040 inches. The score lines should have a depth of from about 0.001-0.060 inches. The disc should moreover be formed from a metal having appropriate characteristics to withstand several downhole conditions, such as stainless steel, nickel or Inconel.